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Thursday, July 24, 2008

Why Microwave Auditory Effect Crowd-Control Gun Won't Work

PHOTO: Franco Vogt/Corbis

22 July 2008—New Scientist magazine recently reported that Sierra Nevada Corp., based in Sparks, Nevada, plans to build what it calls a nonlethal microwave ray gun with the ability to beam irritating sounds into people’s heads. But experts in the underlying biophysics say it cannot work: the device would kill you well before you were bothered by the noise.

The gun, which is being built by Lev Sadovnik at Sierra Nevada, would take advantage of a phenomenon known as the microwave auditory effect. When microwaves are delivered in short pulses, the cochlear tissue in the ear expands. That expansion is heard as an audible click to anyone receiving the radiation, a sound much like that of two rocks being hit together underwater. The company says that the device, called MEDUSA (for “mob excess deterrent using silent audio”), could be used for crowd control.

However, experts say the gun wouldn’t work as advertised. There is no way the ray gun could deliver sound loud enough to be annoying at nonfatal power levels, says Kenneth Foster, a bioengineering professor at the University of Pennsylvania who first published research on the microwave auditory effect in 1974.

“Any kind of exposure you could give to someone that wouldn’t burn them to a crisp would produce a sound too weak to have any effect,” Foster says.

Bill Guy, a former professor at the University of Washington who has also published on the microwave auditory effect, agrees. “There couldn’t possibly be a hazard from the sound, because the heat would get you first,” Guy says.

Guy says that experiments have demonstrated that radiation at 40 microjoules per pulse per square centimeter produces sound at zero decibels, which is just barely in hearing range. To produce sound at 60 decibels, or the sound of normal conversation, requires 40 watts per square centimeter of radiation. “That would kill you pretty fast,” Guy says. Producing an unpleasant sound, at about 120 decibels, would take 40 million W/cm2 of energy. One milliwatt per square centimeter is considered to be the safety threshold.

“There’s a misunderstanding by the public and even some scientists about this auditory effect,” says Guy.

Theoretically, the gun could be used by the military, says James Lin, professor of bioengineering at the University of Illinois. “With any weapon, the intent is to do damage,” he says. In this case, Lin says, the gun would be more likely to cause tissue damage, brain damage, or nerve cell damage than an auditory annoyance.

Sadovnik’s project received a grant from the U.S. Navy Small Business Innovation Research several years ago. The Navy awards grants in phases, and the MEDUSA failed to receive a grant beyond the first phase. Now Sadovnik is working on the project at Sierra Nevada. He declined to comment for this article.